Nicotinamide N-methyltransferase (NNMT) is responsible for the N-methylation of nicotinamide to 1-methylnicotinamide. Our recent studies have demonstrated that NNMT regulates cellular processes fundamental to the correct functioning and survival of the cell. It has been proposed that NNMT may possess β-carboline (BC) N-methyltransferase activity, endogenously and exogenously produced pyridine-containing compounds which, when N-methylated, are potent inhibitors of Complex I and have been proposed to have a role in the pathogenesis of Parkinson's disease. We have investigated the ability of recombinant NNMT to N-methylate norharman (NH) to 2-N-methylnorharman (MeNH). In addition, we have investigated the toxicity of the BC NH, its precursor 1,2,3,4-tetrahydronorharman (THNH) and its N-methylated metabolite MeNH, using our in vitro SH-SY5Y NNMT expression model. Recombinant NNMT demonstrated NH 2N-methyltransferase activity, with a Km of 90 ± 20 µM, a kcat of 3 × 10−4 ± 2 × 10−5 s−1 and a specificity constant (kcat/Km) of 3 ± 1 s−1 M−1. THNH was the least toxic of all three compounds investigated, whereas NH demonstrated the greatest, with no difference observed in terms of cell viability and cell death between NNMT-expressing and non-expressing cells. In NNMT-expressing cells, MeNH increased cell viability and cellular ATP concentration in a dose-dependent manner after 72 and 120 h incubation, an effect that was not observed after 24 h incubation or in non-NNNT-expressing cells at any time point. Taken together, these results suggest that NNMT may be a detoxification pathway for BCs such as NH.
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Cover Image
Cover Image
Crystal structure of human hemoglobin β subunit (PDB ID: 1A3N) with an in silico mutation of phenylalanine 41 to tyrosine (green) to enhance function as a blood substitute; image kindly provide by Brandon Reeder and Chris Cooper (University of Essex). For details see Silkstone et al. in this issue (pages 3371–3383).
Nicotinamide N-methyltransferase catalyses the N-methylation of the endogenous β-carboline norharman: evidence for a novel detoxification pathway
Martin G. Thomas, Davide Sartini, Monica Emanuelli, Matthijs J. van Haren, Nathaniel I. Martin, David M. Mountford, David J. Barlow, Fábio Klamt, David B. Ramsden, Madeehah Reza, Richard B. Parsons; Nicotinamide N-methyltransferase catalyses the N-methylation of the endogenous β-carboline norharman: evidence for a novel detoxification pathway. Biochem J 1 October 2016; 473 (19): 3253–3267. doi: https://doi.org/10.1042/BCJ20160219
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